1. Field of the Invention
The present invention relates to a magnetooptic sensor head that uses Faraday effect of a bismuth-substituted iron garnet single crystal film, and more particularly to a reflection type magnetooptic sensor that uses a bismuth-substituted iron garnet single crystal film having a [111] axis at an angle of 5-60 degrees with an axis normal to the film surface.
2. Prior Art
Today, many of conventional industrial apparatuses and consumer equipment include rotating devices or rotating mechanism such as motors and gears. In order to implement accurate control of rotating apparatuses, the rotational speeds thereof must be measured both continuously and accurately. This measurement requires accurate measuring devices which are simple, light weight, and readily available at low costs and in large quantity.
There have been proposed a variety of methods of measuring rotational speeds. One such method makes use of electromagnetic induction (Sensor Gijutsu, page 68, December, 1986). Another uses a magnetooptic sensor based on Faraday effect of magnetooptic materials (Applied Optics, Vol.28, No.11, page 1,992, 1989). The method based on electromagnetic induction has been used in measuring the rotational speeds of engines for aircraft and automotive vehicles. This type of rotational speed meter is susceptible to electromagnetic noise that comes in through the cables connecting the probe and the main body of the tachometer. Furthermore, since electric circuits are involved, this type of rotational speed meter must be designed so that the electric circuits will not cause explosion when used in the environment where flammable materials such as organic solvents are used or stored.
A magnetooptic sensor head based on Faraday effect of a magnetooptic material, makes use of the changes in rotation of polarization plane of the magnetooptic material in response to the presence and absence of a magnetic field (or a magnet) when a permanent magnet (or magnetic field) approaches the magnetooptic material. That is, the rotation of the polarization plane of a light that is transmitted through a magnetooptic material incorporated in a magnetooptic sensor head, is converted into changes in light intensity, and the number of changes is counted to determine the rotational speed (National Technical Report, Vol.29, No.5, p70, (1983)).
Magnetooptic sensor includes the transmission type and the reflection type. With the transmission type, because of the nature of the structural elements, the elements must be aligned in a straight line so that the light travels straightly. Thus, if some obstructions are located in the course of the light, the magnetooptic sensor head cannot be placed properly.
Meanwhile, Japanese Patent Preliminary Publication No.56-55811 discloses a reflection type magnetooptic sensor head which overcomes the deficiencies of the transmission type magnetooptic sensor head. This magnetooptic sensor has an input light path from which a signal light exits into the Faraday rotator, and an output light path into which the signal light exits from the Faraday rotator. These two light paths are aligned side by side on the same side of the Faraday rotator. In other words, the Faraday rotator is mounted at the tip end of the magnetooptic sensor. Thus, a reflection type magnetooptic sensor head is advantageous in that the sensor head can be installed in a narrow space where a transmission type magnetooptic sensor head cannot be installed.
However, the reflection type sensor head of Matsui et al. is disadvantageous in that one lens must be in series with the polarizer, the other lens must be in series with the analyzer, and these two series connections must be in parallel with each other. This requirement of aligning the series connection side by side places limitations on automatic assembly operation of the entire system in production, and is not cost effective.
FIG. 1 shows Japanese Patent Publication No.3-22595 to Matsumura et al. who propose a configuration where the polarizer and the analyzer are replaced by a single polarizer 5. This configuration overcomes the deficiency of the reflection type magnetooptic sensor head proposed by Matsui et al.
In FIG. 1, the light emitted from a light source 1 such as a semiconductor laser, passes through a lens 2 and a half mirror 3. The light then enters an optical fiber 4. The half mirror 3 permits part of the light incident thereupon to pass through and reflects the remaining light. A photodetector 8 or power meter placed in the light path 9 serves to measure variations in the intensity of light emitted from the light source 1. The signal light directed to the optical fiber 4 passes through a lens 2, half mirror 3 into the optical fiber 4. The signal light exiting the optical fiber 4 passes through the polarizer 5 and the Faraday rotator 6 to a reflecting film 7, which is usually made of a metallic thin film.
The signal light is then reflected by the reflecting film 7 back to the Faraday rotator 6 and then to the polarizer 5. The returning light through the polarizer 5 enters the optical fiber 4. The returning light exiting the optical fiber 4 enters the half mirror 3 which reflects in part the light into the light path 9. The light passing through the light path 9 then enters the photodetector 8 which measures the intensity of the signal light.
Matsumura et al. employed yttrium iron garnet (Y.sub.3 Fe.sub.5 O.sub.12), usually referred to as YIG, as a Faraday rotator produced by flux melt technique. YIG is advantageous as a Faraday rotator element in that Faraday rotation coefficient(deg/cm) is larger in YIG than in paramagnetic glass and zinc selenide. The use of YIG proposed by Matsumura et al. is one way of overcoming the deficiency of a reflection type magnetooptic sensor head proposed by Matsui et al.
In fact, the use of YIG is of great interest as a Faraday rotator element. However, YIG may not be practical as a Faraday rotator since it is well known that YIG transmits lights in near infrared rays having wavelengths longer than 1.1 .mu.m and absorbs lights in 0.8 .mu.m band.
Conventionally, an optical sensor head uses a light source such as semiconductor laser (LD) or light emitting diodes (LED). These light sources have median wavelengths in the range of 0.78-0.85 .mu.m. Semiconductor laser and light emitting diodes are used as a light source for an optical sensor because they are very inexpensive in the above wavelength range as well as photodetectors have good sensitivity in that range. Using light sources available on the market is most preferred and is the best way to provide inexpensive magnetooptic sensor heads in order to meet the User's needs.
High light absorption of YIG in the 0.8 .mu.m band implies that the detection of light may be difficult if a light source available on the market is used. That is, YIG is inherently deficient as a Faraday rotator.
The inventors of the present invention investigated many other materials in order to overcome the deficiency of YIG. The inventors concluded that bismuth-substituted iron garnets could be used as a magnetooptic material. The bismuth-substituted iron garnets can be manufactured rather easily by LPE (Liquid Phase Epitaxial) method, and lends itself to mass production. Bismuth-substituted iron garnets are represented by a chemical formula (RBi).sub.3 (FeA).sub.5 O.sub.12, where R represents yttrium Y or rare earth elements and A represents aluminum Al and gallium Ga.
The Faraday rotation coefficient of a bismuth-substituted iron garnet, i.e., the rotation angle of the polarization plane per unit film thickness at saturated magnetization is as large as several times that of YIG, and more specifically about ten times at 0.8 .mu.m band. This indicates that the film thickness can be smaller with increasing Faraday rotation coefficient for the same magnetooptic effect, achieving less light absorption loss and smaller size. The film thickness of an element can be smaller in bismuth-substituted iron garnets than in YIG, indicating less light absorption. Thus, bismuth-substituted iron garnets are useful in implementing an magnetooptic sensor head with a light source having a wavelength of 0.8 .mu.m band.
The magnetic saturation of bismuth-substituted iron garnets ranges from 500 to 1200 Oe which are about half that of YIG (about 1800 Oe). This indicates that the bismuth-substituted iron garnets can be used to measure weak magnetic fields as well. The ability to measure weak magnetic fields implies that the absence and presence of the magnetic field can be detected even if the magnetooptic sensor head is located far away from a magnet. This provides more flexibility and higher degrees of freedom in installing the magnetooptic sensor head and suggests wider fields of application for magnetooptic sensor heads.
With the aforementioned investigation, the inventors of the present invention believed that reflection type magnetooptic sensor heads can be developed by the use of a bismuth-substituted iron garnet as a Faraday rotator. Based on the disclosure in Japanese Patent Publication No.3-22595, the inventors of the present invention built an engineering model of a reflection type magnetooptic sensor head as shown in FIG. 1 using a Faraday rotator made of a bismuth-substituted iron garnet single crystal in place of YIG.
Then, using the thus built reflection type magnetooptic sensor head, the inventors made a variety of experiments for various magnetic field intensities. However, the sensor head failed to detect any light signal regardless of whether the sensor head is applied with a magnetic field.
Therefore, the inventors made further various experiments in order to find out the reason why the reflection type magnetooptic sensor head according to FIG. 1 failed to detect the light signals. Having made great many experiments, the inventors finally realized that the sensor head failed to detect light due to the magnetic domain structure of the Faraday rotator. The inventors realized that a reflection type magnetooptic sensor head of the construction in FIG. 1 cannot detect light signals if the Faraday rotator is-made of a multidomain element such as bismuth-substituted iron garnets, which have a number of magnetic domains.
The result obtained by the inventors of the present invention do not agree to the experimental results obtained by Matsumura et al. who used YIG as a Faraday rotator which also has multidomain structure as in a bismuth-substituted iron garnet. The bismuth-substituted iron garnet with multidomain did not properly function as a Faraday rotator in the reflection type magnetooptic sensor head built according to the construction of FIG. 1, similar to the YIG in Japanese Patent Publication No.3-22595, while a YIG having the same multidomain functioned properly as a Faraday rotator in Japanese Patent Publication No. 3-22595.
Having reviewed the aforementioned experimental results and making further basic experiments, the inventors confirmed that a reflection type magnetooptic sensor head can be constructed of a reflecting film, (111) bismuth-substituted iron garnet single crystal, polarizer, and light-inputting/outputting paths. Further, the light-inputting/outputting paths are divided into two light paths; an incoming-light path for the light coming into the polarizer from a light source and an outgoing light path for the light leaving the polarizer back to the light source. The two light paths are aligned such that they make an angle greater than 5 degrees with respect to each other. The inventors further continued the research work of magnetooptic sensor heads, and then developed a reflection type magnetooptic sensor head using a Faraday rotator made of a bismuth-substituted iron garnet as disclosed in Japanese Patent Application No.4-90976 (filed on Apr. 10, 1992.).
FIG. 2 shows the construction of a reflection type magnetooptic sensor head disclosed in Japanese Patent Application No. 4-90976. In FIG. 2, a polarizer is depicted at 10. A Faraday rotator 11 is made of a (111) bismuth-substituted iron garnet single crystal film which is magnetized most easily in a direction normal to the film surface. The Faraday rotator 11 is exposed to a magnetic field to be measured. A reflecting film 12 reflects the incident light. An optical waveguide 13 for incoming lights is formed on glass or polymer, or is in the form of an optical fiber. An optical waveguide 14 for outgoing lights is formed on glass or polymer, or is in the form of an optical fiber.
In FIG. 2, the light emitted from a light source 16 such as a semiconductor laser, is directed through a lens 15 into the incoming light path iS. The incoming light path 13 may be directly connected with the light source 16 by omitting the lens 15. The light exiting the light path 13 then passes through the polarizer 10, the Faraday rotator 11 to incident upon the reflecting film 12. The light is then reflected by the reflecting film 12 back through the Faraday rotator 11, the polarizer 10, the light path 14 to enter a photodetector 17 which detects the light as a light signal. With the reflection type magnetooptic sensor head in FIG. 2, the light inputting/outputting port has two independent paths 13 and 14 which make an angle .alpha. greater than 5 degrees relative to each other.
The aforementioned reflection type magnetooptic sensors head using a Faraday rotator made of a bismuth-substituted iron garnet, adequately meet the requirements for a magnetooptic sensor head. However, they still need further many technical improvements. For example, the two light paths must be aligned such that they make an angle .alpha. greater than 5 degrees with respect to each other (Japanese Patent Application No.4-90976), the Faraday rotator must be arranged such that the Faraday rotator is at an angle with an axis normal to the polarizer and reflecting mirror (Japanese Patent Application No.4-116141, filed on May 8, 1992), and the sensor head must be in a unitary construction such that the (111) bismuth-substituted iron garnet single crystal film is sandwiched between slanting surfaces of two rectangular prism (Japanese Patent Application No.4-130674, filed on May 22, 1992). This construction is disadvantageous in implementing a sensor probe having a diameter less than 5 millimeters. Thus, the sensor is not useable for measuring a magnetic field in a very narrow space such as a cylinder provided in the rotating shafts of gyros or turbines where the diameters are on the order of several millimeters. The Faraday rotator is usually cut from an ingot such that [111] axis is normal to the surface of the Faraday rotator as shown in FIG. 7.